Label	Element	Value
Retained Earnings [Member]
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	amg_TaxCutsandJobsActReclassificationfromAOCItoRetainedEarningsTaxEffect	$ (6,600,000)

Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect

No definition available.

Name:	amg_TaxCutsandJobsActReclassificationfromAOCItoRetainedEarningsTaxEffect
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration